1. Field of the Invention
This invention relates to a spring brake actuator. More specifically, the invention relates to a spring brake actuator having a hollow actuator rod and an improved anchor for mounting a reaction plate to the actuator rod.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of a fluid such as compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency or spring brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake. Typically, the spring brake actuator is disposed in tandem with the service brake actuator.
When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm and a pressure plate compresses the spring. In many applications, a spring brake actuator rod is held in a retracted position by a relatively small return spring. In newer applications, the spring brake actuator rod is integral with the pressure plate and held in a retracted position by the air pressure. In both designs, the spring brake actuator rod thus does not affect the normal operation of the brake.
Depressing the brake pedal during normal driving operation introduces compressed air into the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and the brakes to be applied with an application force proportional to the air pressure in the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the strong compression spring acting on the spring brake actuator rod which, in turn, acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
U.S. Pat. No. 5,105,727 to Bowyer, issued Apr. 21, 1992, discloses a known spring brake assembly which includes both a spring brake portion and a service brake portion. The service brake portion includes an air chamber partially defined by a flexible diaphragm acting against a service brake push rod and a return spring to assure proper release of the brake when air is exhausted from the air chamber. The spring brake portion also includes an air chamber comprising a spring brake housing and a spring brake actuating rod formed integral with a spring brake pressure plate and secured to the spring brake diaphragm. The spring brake diaphragm is provided with a centrally disposed aperture and engages the actuating rod and pressure plate to form an airtight seal. A strong power spring acts between an end of the spring brake housing and the pressure plate, and tends to urge the actuating rod out of the air chamber to engage the diaphragm and push rod of the service brake and to cause the brake to be applied. During normal operation, the spring force is counteracted by air pressure inside of the spring brake air chamber which acts against the diaphragm to compress the power spring, retracting the actuating rod. When system air pressure is lost, or manually released, the power spring extends to apply the brakes.
The actuating rod is hollow and provided with a central bore. The proximal end (nearest the spring brake housing) of the actuating rod is partially closed by an inwardly directed annular flange on a pressure plate. A brake releasing caging bolt extends into the central bore from outside of the spring brake housing and has an end plate for engaging the annular flange. The bolt is threaded and extends out of the actuating rod, through an aperture in the spring brake housing, through a threaded nut, and terminates in a bolt head.
The opposite end of the actuator rod in the service chamber terminates in a reaction plate. In brake actuators employing solid actuator rods, it is known to attach the reaction plate to the actuator rod by welding or by threading a bolt through an aperture in the reaction plate and into a threaded hole in the end of the actuator rod. A problem, however, is present in connecting a hollow actuator rod, such as that disclosed in the Bowyer U.S. Pat. No. 5,105,727, to the reaction plate.